ASoC: cs35l35: Add DT binding to specify usage of an external boost supply

Add a device tree binding to let the driver know that the amplifier is
configured to use an external boost supply.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Acked-by: Brian Austin <brian.austin@cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>